Burning of fossil fuels is thought to have resulted in elevated atmospheric carbon dioxide (CO2) concentrations. The levels of carbon dioxide are expected to double in as little as 60 years based on changes in land use and continued burning of fossil fuels. The increase in carbon dioxide concentrations as well as other greenhouse gases is thought to keep heat within the atmosphere, leading to higher global temperatures. Sequestration—the long term capture and storage of carbon dioxide—has been long thought of as a way to mitigate this problem. Given however, that light and carbon dioxide make up most of what is consumed, direct conversion of ambient carbon dioxide to valuable products, such as fuels, chemicals, drugs, and their precursors, represents an alternative and improved means to reduce the effects of carbon dioxide while maintaining the core industrial and commercial products our modern society demands.
Plants and other light capturing organisms are the main method by which carbon dioxide is removed from the atmosphere. Through photosynthesis, organisms use solar energy while capturing carbon dioxide, important metabolic precursors can be made that can be converted to biomass in amounts exceeding 90% (Sheehan John, Dunahay Terri, Benemann John R., Roessler Paul, “A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae,” 1998, NERL/TP-580-24190). Previous approaches have sought to increase production of algal biomass and potentially use that biomass as a fuel. (Reed T. B. and Gaur S. “A Survey of Biomass Gasification” NREL, 2001). It has been additionally demonstrated that addition of a small subset of genes can enable light capturing organisms to produce ethanol. Specifically, the expression of alcohol dehydrogenase II and pyruvate decarboxylase from Z. mobilis in a Cyanobacterium has been achieved resulting in low levels of ethanol production (U.S. Pat. No. 6,699,696). Nonetheless, the ability to produce algae as well as to produce products from light capturing organisms has been well below the efficiency needed to have a commercially viable and therefore meaningful impact on ambient or waste carbon dioxide (U.S. Pat. No. 6,699,696; Sheehan John, Dunahay Terri, Benemann John R., Roessler Paul, “A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae,” 1998, NERL/TP-580-24190)
One of the primary limitations of using algae as a method of carbon dioxide sequestration or conversion to products has been the development of efficient and cost-effective growth systems. Aquatic organisms, such as algae, oysters, and lobsters, have been primarily cultured in open systems. This approach allows for the organisms to take advantage of the semi-natural environment while keeping operational expenditures potentially lower. Open algal ponds up to 4 km2 have been researched, which, while requiring low capital expenditures, ultimately have low productivity as these systems are also subject to a number of problems. Intrinsic to being an open system, the cultured organisms are exposed to a number of exogenous organisms which may be symbiotic, competitive, or pathogenic. Symbiotic organisms can change the culture organisms merely by exposing them to a different set of conditions. Opportunistic species may compete with the desired organism for space, nutrients, etc. Additionally, pathogenic invaders may feed on or kill the desired organism. In addition to these complicating factors, open systems are difficult to insulate from environmental changes including temperature, turbidity, pH, salinity, and exposure to the sun. These difficulties point to the need to develop a closed, controllable system for the growth of algae and similar organisms.
Not surprisingly, a number of closed photobioreactors have been developed. Typically, these are cylindrical or tubular (i.e., U.S. Pat. No. 5,958,761, US Patent application No. 2007/0048859). These bioreactors often require mixing devices, increasing cost, and are prone to accumulating oxygen (O2), which inhibits algal growth.
As discussed in WO 2007/011343, many conventional photobioreactors comprise cylindrical algal photobioreactors that can be categorized as either “bubble columns” or “air lift reactors.” Vertical photobioreactors, which operate as “bubble columns” are large diameter columns with algal suspensions wherein gas is bubbled in from the bottom. Using bubbling as a means of mixing in large-diameter columns is thought to be inefficient, providing for lower net productivity as certain elements of the culture remain photo-poor and as large bubbles of gas do not deliver necessary precursors. An alternative vertical reactor is the air-lift bioreactor, where two concentric tubular containers are used with air bubbled in the bottom of the inner tube, which is opaque. The pressure causes upward flow in the inner tube and downward in the outer portion, which is of translucent make. These reactors have better mass transfer coefficients and algal productivity than other reactors, though controlling the flow remains a difficulty. Efficient mixing and gas distribution are key issues in developing closed bioreactors and to date, such efficient bioreactors do not exist.
Tubular bioreactors, when oriented horizontally, typically require additional energy to provide mixing (e.g., pumps), thus adding significant capital and operational expense. In this orientation, the O2 produced by photosynthesis can readily become trapped in the system, thus causing a significant reduction in algal proliferation. Other known photobioreactors are oriented vertically and agitated pneumatically. Many such photobioreactors operate as “bubble columns.”
All closed bioreactors also require light, either from the sun or artificially derived (U.S. Pat. No. 6,083,740). Solar penetration is typically enabled through translucent tubing, which, with thinner diameter, enables more thorough saturation of the algae. Some known photobioreactor designs rely on artificial lighting, e.g. fluorescent lamps, (such as described by Kodo et al. in U.S. Pat. No. 6,083,740), and can otherwise be provided by any light source existing today. Photobioreactors that do not utilize solar energy but instead rely solely on artificial light sources can require enormous energy input, increasing cost, and rendering these systems, as stand-alone approaches, impractical. Using natural solar light requires a low cost means to allow for proper penetration of the culture while maintaining the culture at a temperature that is appropriate.
In addition, because of geometric design constraints, during large-scale, outdoor algal production, both types of cylindrical photobioreactors can suffer from low productivity, due to factors related to light reflection and auto-shading effects (in which one column is shading the other). Shading issues make for inefficiencies on vertical bioreactor design, leading to low land use.
Several flat-plate photobioreactor designs have been disclosed for culturing microalgae: Samson R & Leduy A (1985) Multistage continuous cultivation of blue-green alga Spirulina maxima in the flat tank photobioreactors with recycle. Can. J. Chem. Eng. 63: 105-112; Ramos de Ortega and Roux J. C. (1986) Production of Chlorella biomass in different types of flat bioreactors in temperate zones. Biomass 10: 141-156; Tredici M. R. and Materassi R. (1992) From open ponds to vertical alveolar panels: the Italian experience in the development of reactors for the mass cultivation of photoautotrophic microorganisms. J. Appl. Phycol. 4: 221-31. Tredici M. R., Carlozzi P., Zittelli G. C. and Materassi R. (1991) A vertical alveolar panel (VAP) for outdoor mass cultivation of microalgae and Cyanobacteria. Bioresource Technol. 38: 153-159; Hu Q. and Richmond A. (1996) Productivity and photosynthetic efficiency of Spirulina platensis as affected by light intensity, algal density and rate of mixing in a flat plate photobioreactor. J. Appl. Phycol. 8: 139-145; Hu Q, Yair Z. and Richmond A. (1998) Combined effects of light intensity, light-path and culture density on output rate of Spirulina platensis (Cyanobacteria). European Journal of Phycology 33: 165-171; Hu et al. WO 2007/098150, however, to date, no design or system has been successfully scaled up for efficient growth of organisms in commercial scale.
Many different photobioreactor configurations have been described in the literature including flat panels, bubble columns, tubular reactors and a variety of annular designs aimed at improving the surface area to volume ratio to maximize conversion of sunlight and CO2 to biomass or other products such as algal oil. These reactors have distinct advantages compared to open raceway with respect to controlling temperature, pH, nutrient and limiting contamination (see Pulz, O. “Photobioreactors: Production systems for phototrophic microorganisms”, Appl. Microbiol. Biotechnol (2001) 57:287-293). Key limitations to their adoption have been the cost vs. benefit as it relates to the product being produced. Whereas valuable products such as carotenoids have been produced in photobioreactors the production of biomass for fuels could not be economically justified to date.
The art as it relates to enclosed photobioreactors achieve temperature control in a variety of ways including external and internal heat exchangers, spraying of cooling water directly on the surface, use of cooled or heater sparge gas as well as submerging the reactor directly in large pond of water that is separately temperature controlled (see Molina Grima, E. et al “Photobioreactors: light regime, mass transfer, and scale-up”, J. of Biotechnology (1999) 70:231-247; Hu, Q. et al “A flat inclined photobioreactor for outdoor mass cultivation of photoautotrophs” Biotechnology and Bioengineering (1996) 51:51-60 and Hu, Q. WO 2007/098150 A2 “Photobioreactor and uses therefor”). Currently, a cost-effective thermal regulation system that can be implemented in large scale does not exist.
What is needed, therefore, is an integrated photobioreactor system that is scalable, low cost, and efficient for culturing light-capturing organisms.